# How to Find the Sum of Prime-Indexed Elements in Go

## The challenge

You will be given an integer array and your task is to return the sum of elements occupying prime-numbered indices.

The first element of the array is at index “.

## The solution in Golang

Option 1:

```
package solution
import "math/big"
func Solve(arr []int) int {
c := 0
for i := 0; i < len(arr); i++ {
n := arr[i]
if big.NewInt(int64(i)).ProbablyPrime(0) {
c += n
}
}
return c
}
```

Option 2:

```
package solution
func Solve(arr []int) int {
var sieve = make([]bool, len(arr))
count := 0
for i := 2; i < len(arr); i++ {
if !sieve[i] {
count += arr[i]
for j := i * i; j < len(sieve); j += i {
sieve[j] = true
}
}
}
return count
}
```

Option 3:

```
package solution
func Solve(arr []int) int {
count := 0
sum := 0
for i := 2; i < len(arr); i++{
for c, n := i, 2; c > 0; n++{
if i % n == 0{
count++
c /= n
n--
}
}
if count < 3 {
sum += arr[i]
}
count = 0
}
return sum
}
```

## Test cases to validate our solution

```
package solution_test
import (
. "github.com/onsi/ginkgo"
. "github.com/onsi/gomega"
)
func dotest(arr []int, exp int) {
var ans = Solve(arr)
Expect(ans).To(Equal(exp))
}
var _ = Describe("Example tests", func() {
It("It should work for basic tests", func() {
dotest([]int {}, 0)
dotest([]int {1,2,3,4},7)
dotest([]int {1,2,3,4,5,6}, 13)
dotest([]int {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15},47)
})
})
```